The purpose of the proposed project is to identify the neurons, and characterize the segmental input to the central timing network (CTN) responsible for producing the cycle-to-cycle timing of the turtle swimming rhythm. Recent studies have indicated that electrical stimulation of segmental afferents, travelling in muscle-nerves, can alter the locomotor rhythm by phase shifting the CTN. A series of experiments on decerebrated turtles will combine electrically elicited swimming with controlled stretches of hindlimb muscles to ask three questions: 1) Can transient muscle length changes modify the time of protraction and retraction transitions during the limb movement cycle?; 2) Can afferent input resulting from cyclically recurring changes in muscle length entrain and stabilize the locomotor rhythm?; and 3) Do imposed length changes in different muscles yield quantitatively or qualitatively distinguishable locomotor modifications which can be correlated with the functional role or anatomical position of the muscle? Findings will contribute to our understanding of peripheral feedback to the CTN during locomotion. A separate series of intracellular studies will attempt to identify, characterize and record activity in CTN neurons. These studies will take advantage of the fact that the turtle represents the only limbed vertebrate in which an experimentally feasible set of criteria may be available for determining that a single interneuron is part of the central locomotor network. The clinical importance of the proposed project lies in an expansion of our understanding of the interactions between the CNS and afferent feedback during motor behaviors. In particular, clinical assessment and rehabilitation of CNS injured patients might benefit from more specific information regarding peripheral inputs which directly influence the rhythmic output during locomotion.